Here's something you hardly ever see anymore. The House and Senate just passed two bills to promote clean energy around the United States. Not only that, but the votes were virtually unanimous.

So what's this mysterious energy source attracting all-but-unheard-of bipartisan affection? Hydropower.

Dams already provide the largest source of renewable energy* in the United States — they provide about 7 percent of America's electricity and account for fully two-thirds of the country's renewable power, dwarfing wind or solar.

But even though most of the big, wild rivers out West have already been dammed up, there are still plenty of places to eke out more power from smaller rivers, streams, and canals. And that's where Congress comes in.

--Small hydropower projects. The first place to look is the 80,000 dams in the United States that already exist but don't yet provide power. A study (pdf) last year by the Department of Energy suggested that the country could add up to 12,000 MW of capacity by retrofitting the largest of these dams, which would boost the country's total hydropower capacity by up to 15 percent. That's like adding a dozen large coal-fired plants:

At the high end, adding all of those small projects could boost the share of carbon-free electricity in the United States by about 1 percent. That's obviously not going to stop global warming on its own, but it's a small dent.

So the first of the House (H. 267) and Senate (S. 545) bills would speed up the licensing time for small projects like these. Both of these bills passed unanimously and had sponsors from both parties — Reps. Cathy McMorris Rodgers (R-Wash.) and Diana DeGette (D-Colo.) and Sens. Ron Wyden (D-Ore.) and Lisa Murkowski (R-Alaska).

One caveat: It's not quite clear if all 12,000 megawatts of capacity will actually get built under this bill, says Matthew Nocella, a spokesman for the National Hydropower Association. "It will be much more feasible for developers if they can go through the process in two years as opposed to six," he said. "But there are no studies on what the exact impact will be."

--Clearing up regulatory hurdles. The other set of bills passed by Congress (H.R. 678 and S. 306) would make it easier for the Bureau of Reclamation to add hydropower to many of the canals, pipelines, aqueducts, and other man-made waterways that it already owns. In the Senate, these bills were sponsored by Sens. John Barrasso (R-Wyo.) and Jim Risch (R-Idaho).

The Bureau had put out a report (pdf) last year suggesting that it could add at least 268 megawatts of hydropower capacity if it could develop these sites. Until now, however, the agency had been bogged down by regulatory delays.

So why does hydropower enjoy such wide support at a time when energy legislation typically gets bogged down in partisan feuding? For one, dams have always been popular, particularly out West. But it helped that these bills were quite modest, mostly clearing up government red tape and bolstering development on already-existing dams (which helped mute much of the environmental opposition).

--A hydropower renaissance? In theory, Congress could still do more to promote hydropower. A study in 2010 from Navigant Consulting found that some 60,000 MW of capacity could be added with the right policies. But that study assumed Congress would pass a law that required utilities to get a big chunk of their electricity from renewable sources. That's much more contentious.

A major hydropower push would also likely require new technologies, something that Wyden and Murkowski are now promoting with their "Marine and Hydrokinetic Renewable Energy Act," which would fund research into ways to tap energy from rivers and streams that don't involve damming up their flow and creating large reservoirs.

Also note that support for hydropower is hardly universal outside of Congress. Back in 2011, my colleague Juliet Eilperin reported on how some environmental groups and Native American tribes were pushing to demolish a number of dams in the Pacific Northwest — including ones that provided power — in order to rescue depleted fisheries and restore natural habitats. So there's a lot of potential for push-back.

For now, though, hydro supporters were happy to tout their victory — however modest. "There’s no better evidence that hydro is back than these two bills passing the Senate on a unanimous vote,” Wyden said.

* Clarification: Sorry, bad wording. Dams are the largest source of renewable electricity in the United States, providing 7 percent of total electricity. Nuclear power, of course, provides the largest source of carbon-free electricity, generating about 19 percent of total electricity.